While running, heavy-load pneumatic tires for trucks, busses and the like, or off-road tires, especially the rubber composition arranged in the tread part thereof are repeatedly given a great strain amplitude, and the tire temperature is thereby turned up. Consequently, for prolonging the life of tires, the tire temperature increase must be prevented. For preventing the tire temperature increase, it is desirable that the tire tread rubber is prevented from being deformed by increasing the elastic modulus thereof in a high-temperature range, and/or the low-heat-generation property of the tire tread rubber composition is improved, or that is, tan δ thereof is lowered.
In already-existing pneumatic tires, the amount of the filler such as carbon black or the like in a rubber composition is increased or an additive such as a resin or the like is added to the composition for improving the elastic modulus of the composition. In this case, however, there occur problems in that the viscosity of the unvulcanized rubber composition increases owing to the increase in the amount of the filler so that the workability during kneading of the composition worsens, or tan δ of the vulcanized rubber composition increases.
On the other hand, for lowering tan δ, the type and the amount of the carbon black to be in the rubber composition are changed, for example, the particle size of the carbon black is increased or the amount of the carbon black is decreased. However, these methods lower the abrasion resistance of the tread rubber and lower the fracture resistance such as cutting resistance and chipping resistance thereof, and after all, it is difficult to improve the low-heat-generation property of the rubber composition by changing the type and the amount of carbon black to be in the composition. In addition, it is known that, when carbon black is substituted with silica and is used along with a silane coupling agent, then tan δ can be reduced; however, the abrasion resistance and the fracture resistance of the rubber composition are lowered and the durability of tires is thereby worsened.
As opposed to this, in PTL 1, it has been found that addition of a hydrazide compound can improve the low-heat-generation property with neither increasing the Mooney viscosity of the resultant composition nor lowering the abrasion resistance and the fracture resistance thereof.
However, it is desired to provide a rubber composition satisfying a high elastic modulus and a low tan δ both on a further higher level.